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Kidney Function Tests
Contents:
• Functional units
• Kidney functions
• Renal diseases
• Routine kidney function tests
• Serum creatinine
• Creatinine clearance
• Cockcroft-Gault formula for GFR estimation
• Serum Urea
Functional units :
 The nephron is the functional unit of the kidney
 Each kidney contains about 1,000,000 to
1,300,000 nephrons.
 The nephron is composed of glomerulus and
renal tubules.
 The nephron performs its homeostatic function
by ultra filtration at glomerulus and secretion
and reabsorption at renal tubules.
Representation of a nephron and its blood supply
Kidney functions :

Regulation of :
- water and electrolyte balance.
- acid base balance.
- arterial blood pressure.

Excretion of metabolic waste products and
foreign chemicals.

Hormonal Function: Secretion of
erythropoietin & activation of vitamin D and
activation of angiotensinogen by renin

Metabolic Function: site for gluconeogenesis
Renal diseases:
 Many diseases affect renal function.
 In some, several functions are affected.
 In others, there is selective impairment of
glomerular function or one or more of tubular
functions.
 Most types of renal diseases cause
destruction of complete nephron.
Routine kidney function test include the
measurement of :
 Serum creatinine.
 Creatinine clearance.
 Serum urea.
Both serum creatinine and creatinine clearance
are used as kidney function tests to :
 Confirm the diagnosis of renal disease.
 Give an idea about the severity of the disease.
 Follow up the treatment.
Serum creatinine (55-120 mol/L in adult):
• Creatinine is the end product of creatine catabolism.
• 98% of the body creatine is present in the muscles
where it functions as store of high energy in the form
of creatine phosphate.
• About 1-2 % of total muscle creatine or creatine
phosphate pool is converted daily to creatinine
through the spontaneous, non enzymatic loss of water
or phosphate.
• Creatinine in the plasma is filtered freely at the
glomerulus and secreted by renal tubules (10 % of
urinary creatinine).
• Creatinine is not reabsorbed by the renal tubules.
• Plasma creatinine is an endogenous substance not
affected by diet.
• Plasma creatinine remains fairly constant throughout
adult life.
Creatinine clearance :
• The glomerular filtration rate (GFR) provides a useful
index of the number of functioning glomeruli.
• It gives an estimation of the degree of renal
impairment by disease.
Accurate measurement of GRF by
clearance tests requires determination
of the concentration in plasma and urine
of a substance that is:
• Freely filtered at glomeruli.
• Neither reabsorbed nor secreted by tubules.
• Its concentration in plasma needs to remains constant
throughout the period of urine collection.
• Better if the substance is present endogenously.
• Easily measured.
Creatinine meets most of these criteria.
• Creatinine clearance is usually about 110 ml/min in the
20-40 year old adults.
• It falls slowly but progressively to about 70 ml/min in
individuals over 8o years of age.
• In children, the GFR should be related to surface area,
when this is done, results are similar to those found in
young adults.
• Clearance is the volume of plasma cleared from
the substance excreted in urine per minute.
• It could be calculated from the following
equation:
Clearance (ml/min) = U  V
P
U = Concentration of creatinine in urine mol/l
V = Volume of urine per min
P = Concentration of creatinine in serum mol/l
Cockcroft-Gault Formula
for Estimation of GFR

As indicated above, the creatinine clearance is
measured by using a 24-hour urine collection, but this
does introduce the potential for errors in terms of
completion of the collection.

An alternative and convenient method is to employ
various formulae devised to calculate creatinine
clearance using parameters such as serum creatinine
level, sex, age, and weight of the subject.

An example is the Cockcroft-Gault Formula:
GFR =
K  (140 – age)  Body weight
──────────────────
Serum creatinine (mol/L)

where K is a constant that varies with sex:
1.23 for male & 1.04 for females.

The constant K is used as females have a relatively
lower muscle mass.
Cockcroft-Gault Formula
for Estimation of GFR: Limitations

It should not be used if
Serum creatinine is changing rapidly
the diet is unusual, e.g., strict vegetarian
Low muscle mass, e.g., muscle wasting
Obesity
Serum creatinine is a better kidney function test
than creatinine clearance because :
• Serum creatinine is more accurate.
• Serum creatinine level is constant throughout adult life
Creatinine clearance is only recommended in the
following conditions:
• Patients with early ( minor ) renal disease.
• Assessment of possible kidney donors.
• Detection of renal toxicity of some nephrotoxic drugs.
Normal adult reference values:
Urinary excretion of creatinine is 0.5 - 2.0 g per 24 hours in a normal
adult, varying according to muscular weight.
- Serum creatinine :
55 – 120 mol/L
- Creatinine clearance: 90 – 140 ml/min
80 – 125 ml/min
(Males)
(Females)
A raised serum creatinine is
a good indicator of impaired renal function
But normal serum creatinine
does not necessarily indicate normal renal function as
serum creatinine may not be elevated until GFR has fallen
by as much as 50%
Serum Urea ( 2.5-6.6 mmol/L) in adult:
Urea is formed in the liver from ammonia released
from deamination of amino acids.
As a kidney function test, serum urea is inferior
to serum creatinine because:
 High protein diet increases urea formation.
 Any condition of  proteins catabolism (Cushing
syndrome, diabetes mellitus, starvation,
thyrotoxicosis)  urea formation.
 50 % or more of urea filtered at the glomerulus is
passively reabsorbed by the renal tubules.
Normal values of Internal Chemical Environment
controlled by the Kidneys:
SODIUM
135 to 145 mEq/L
POTASSIUM
3.5 to 5.5 mEq/L
CHLORIDES
100 to 110
BICARBONATE
24 to 26 mEq/L
CALCIUM
8.6 to 10 mg/dl
MAGNESIUM
1.6 to 2.4 mg/dl
PHOSPHORUS
3.0 to 5.0 mg/dl
URIC ACID
2.5 to 6.0 mg/dl
pH
7.4
CREATININE
0.8 to 1.4 mg/dl
BUN (Blood Urea Nitrogen)
mEq/L
15 to 20 mg/dl